I'm trying to disable ipv6 or use firewall on a mikrotik switch (7.6 OS) I have in network.
My computer is connected though cable to this switch, switch is connected through cable to mikrotik router (7.6 OS). There is a linux os connected to router as well.

I can ssh or ping that linux using ipv6.
Now, if I add drop action to forward chain on switch's ipv6 firewall, linux is still accessible using its ipv6 address.
If I disable ipv6 (ipv6/settings) or try to disable any other settings, ssh and ping still work.

I'm sure I'm missing something really basic....

Nevermind, firewall doesn't work within same subnet (while bridge filters do) and it doesn't matter whether it is enabled on switch or now I guess.

That is correct. You do not need IPv6 enabled for a switch or bridge to pass IPv6 traffic within a LAN segment (subnet).