No.
Only the data partition gets encrypted. OS updates don’t affect the data partition.
(I know Android says that it encrypts “the phone”. That’s a lie 
.)
If TWRP can’t decrypt the data partition, the biggest catch will be that you will not have the data partition in a TWRP backup.
And there’s of course some smaller stuff, e.g. you obviously can’t have a look at the data partition with the TWRP file manager this way.
But apart from that, TWRP will still work for installs and updates and backups of the system and cache partitions.