The numbers should show only when the Alt key is pressed?

Well, it seems that it depends on design decision.

IMO, it is better to keep showing the number.

For me pressing Alt with a number does nothing. Is this locale-dependent? I use Czech locale, so numbers generally require holding Shift too (but usually not for keyboard shortcuts). I tried even with Shift and still no effect.

I tried using a default Czech keyboard layout and can confirm that Alt + number does not change tabs because in this layout the numbers need a shift key. It is known system (or possible upstream) issue that shortcuts containing Alt + Shift do not work. See elementary/gala#1261.

I see. Note that I can for example switch workspaces with Super + a number. (Except Super+1 works only sometimes as sometimes it gets assigned as Super++ after reboot and then it does zoom instead. Some race condition there, but no idea how to test it.)

Hmm, strange, the Gala shortcuts with Super seem to work differently - perhaps relying on hardware keycodes or something (?) For example Super + + with gb layout (which is Super + Shift + the 13th key on the second row) does nothing whereas Super + + with cz layout (which is Super + the 2nd key on the second row) zooms in. Zoom in with gb layout is Super + = (which is Super + the 13th key on the second row).

@janxkoci Try changing the layout change shortcut in System Settings/Keyboard/Layout to (e.g. "Both Shift keys together"). I find that Terminal will then switch tabs as expected using the cz layout because Alt + Shift is not being intercepted by the system.

@jeremypw interesting, that makes it work, thanks. I don't really use this to switch tabs (from other terminals I'm used to using Ctrl+PgUp & Ctrl+PgDn, as they are close to the right Ctrl key on my keyboards, but this doesn't work in elementary Terminal, so I'm now learning Ctrl+Tab and Ctrl+Shift + arrows instead).

Mostly I don't have that many terminals to need numbers... 🤷‍♂️

And sorry, I didn't want to hijack the issue like that..

Another partial solution would be to have Terminal recognise Alt + KP_1 etc for switching tabs (which it should do anyway really) as the number keypad is less likely to be affected by locale.

Looks like fixing the keypad issue has to be done in Granite as it is a bug in the DynamicNotebook widget and also occurs in Code. Granite development is now targeted at elementary 7 and Gtk4 so I cannot do it atm. The Super workspace switching issue seems to be an upstream bug in libmutter rather than the elementary window manager (Gala). The binding of of keys to display operations is done there.