Is using the F3 + T trick to hold a right click to bang a noteblock to avoid getting afk kicked allowed?

There is a clarification on AFK rules here: https://forum.craftymynes.com/10144-afk-rules-clarification
I am no authority but what you are proposing is within the permitted scope as I understand it. I may give it a try myself - never used that method.

I used to do it to run on the nether roof before elytras and no one said it was against the rules so probably allowed but best to just ask an admin.

yes

alr ty