Unimplemented sound systems

Keyboard and rack synthesizers

• Roland linear arithmetic (D-50, MT-32, etc): Preliminary support on MAME.
• NEC uPD933 (Casio phase distortion): Only recently marked "good" on MAME.
• Yamaha's pre-FM digital chips (YM1019, YM1104, YM1105, IG10771): No reverse-engineering efforts or adequate documentation, extremely obscure, mostly high-length wavetable chips.
• Curtis CEM3374 (Zaccaria pinballs ranging from Clown to Spooky, Oberheim Matrix synths): Way more barebones analog dual-VCO that can do triangle and saw. Documented at https://www.synfo.nl/datasheets/CEM3374.pdf. The SID can already do everything that the CEM3374 natively can except for freq mod.
• Curtis CEM3394 and National Semiconductor MM5837 (several analog-subtractive synths, Sente arcade machines): MAME support only recently became mostly complete. Sente games use it more like a 2x SID with per-channel filters but no wave fusion or other digital effects.
• Yamaha DX7: Monotimbral, meaning only one unique instrument is active at a time, and 16 voices only means 16 instances of the same instrument.
• STMicro M114S (Amstrad CKX100, several European (mainly Italian) synthesizers and organs, Zaccaria pinballs as Mr. Game): Iffy emulation and only on PinMAME. Its main selling point is hardware-level wave morphing, which can already be done in the wavetable tab of the instrument editor.
• L7A1045 L6028 DSP-A (Akai MPC3000, Hyper Neo Geo 64): Explained down below.

Computers and consoles

• Atari AMY: 64-voice additive synthesizer with complete docs, but no emulation material.
• Yamaha SCSP (Sega Saturn): Too overwhelming to understand or implement.
• Astrocade PSG: Limited capabilities, and too few potential users. Mostly redundant with the Casio PV-100.
• Mountain Computer Music System: Barebones 16-voice wavetable synth of 8-bit 256-length waves, as a pair of sound cards for the Apple II. Only used in professional music production software, and easily imitated with an ES5503.
• Signetics 2636: Used in the Century Electronics CVS arcade system and a few small home consoles. CVS games use pulse widths and other quirks.
• Yamaha YMF288 OPN3/EPSM: Same as the YM2608, but without the ADPCM or CSM. Any importers or converters from FamiStudio or other sources should target their EPSM data to a YM2608.
• OPP: Differences between OPM and OPP are extremely insignificant.
• RCA CDP1869 (Cidelsa arcade games, Comx-35): The TED, but slightly dumbed-down.

Arcade and pinball

• Stern BSMT2000: A TMS320C15 DSP with a different mask ROM, used as a 12-voice sample chip. Needs a ROM to work.
• Yamaha YMF271: Same reason as SCSP, and also inferior emulation outside of PCM despite being in the VGM spec.
• Capcom CPS3: Probably the same as QSound, or mostly.
• Galaxian custom sound and derivatives like in Sigma's "Spiders!" and SNK6502 games: Explained at Plogue's blog post, while the others are hardwired analog circuits. Pong is the only exception to such thing not being added, and it was added as a joke. Plogue Chipsounds has a few of these systems (Galaxian, Donkey Kong, Zaxxon, Phoenix). The planned patchbay feature (a node editor) could have VST effects (as DSPs) and abstract circuits, making it likely to allow recreating such circuits yourself.
• OKI MSM5205: Planned. A predecessor to the 6258 used in the X68000.
• Williams-Bally DCS sound system: That's a codec, not an actual custom circuitry. The hardware is (initially) an ADSP-2105 processor running an engine that can do up to 4 simultaneous sounds. The music is a single audio stream (as in MP3 or WAV) instead of a MIDI-like sequence, making the game ROM significantly bigger.
• Namco C352, Ricoh RF5C400, Taito ZSG-1, IGS ICS2115, Hyper Neo Geo 64, Yamaha AICA, Eolith QS1000, other >24 voice hi-fi PCM chips: Redundant with the ES5506, but the PlayStation SPU might have features that stand out other than simply being the successor to the SNES. The only exception is the OPL4 (and therefore the MultiPCM) which is only added to complete the series.
• SNK Wave: Equivalent to the Commodore PET, but the wave is 3-bit instead of 1-bit. Easily imitated with a generic DAC or a certain wavetable chip.
• "Exidy SFX" or "Exidy SFX + PSG" (Exidy and Stern games): Mostly an AY-3-8910 but as a PCB, via a Motorola MC6840 PIT for the square waves and a 128-bit Motorola MC14562 shift register for the noise, along with an envelope generator that's mostly seen in Stern pinballs.
• Konami K054539: Inaccurate echo emulation.

Other

• Acorn Archimedes VIDC, Macintosh, Gaelco, Brezzasoft Vrender0, Imagetek I5000: Easily redundant with generic PCM DACs or a YMZ280B, while the Macintosh is specifically redundant with the Amiga Paula and the I5000 is redundant with the QSound.
• Software-based sound DSPs (Atari Jerry, N64, Atari STe, Sega 32X): The GBA and ZX Spectrum are exceptions due to having features specific to them, but these are all CPU-driven softmixed audio. The Jerry is said to have "FM, wavetable, PCM" but those are advertised driver functions. The N64 is said to have 100 PCM channels, but that's theoretical and assumes that each audio stream takes a maximum of 1% processing load. The only exceptions are those that have features that make them stand apart, such as the QSound DSP.
• Yamaha SMAF chips (MA-series): An entire community was invested in emulation and reverse engineering projects, with their furthest progress being manual and low-level interfaces to the chips, such as signal converters from OPLx to MA-x. No reverse-engineering was done on the official emulation cores.
• MT32 / LA32, Sound Canvas, other MIDI modules: Furnace is not intended to be a tracker for any MIDI devices, rather being intended for being able to directly drive sound chips. There are multiple issues with implementing devices like these, such as lack of control over what the hardware does, along with requiring copyrighted data and code. If adequate FOSS Yamaha SMAF emulation exists, It's the only reason why MMF export will never be added without the resulting file being way more limited and having way less features, as the low-level chip controls break compatibility with the inherently high-level MMF format that's meant to be MIDI but with custom patches instead of being tied to a hardcoded instrument library.
• Lexaloffle fantasy machines (PICO-8, Voxatron, Picotron): Lack of control over sound, same as MIDI in this regard.
• KR580VI53 (a 3-channel sound system using Soviet clone of Intel 8253 timer found in Vector-06C): can easily be imitated by using 3 PC speakers.