You can use :erlang.append_element(tuple, value) to skip the Elixir wrapper. It does not seem to be scheduled for deprecation in OTP 28 or 29.
Now the Elixir commit that added the deprecation seems to be related with types, so I guess you will not benefit from them if you do that.
So I would use tuple_size/1. Honestly I have never needed to append to a tuple anyway 
And thank you to the team! Great achievements this year \o/.
