Historically, many SE tags have been inspired from Monero source code (see monero-wallet-cli, monero-wallet-gui, txpool and other examples).
I followed the conversation related to this pull request on Reddit and similarly would like to rename mixin to ring size ring-size on Stack Exchange.
The pull request referenced above only encompasses a visual change in the GUI aimed at making Monero terminology easier to understand for those that may misinterpret the meaning of mixin.
Before proposing a more drastic pull request removing mixin from all Monero source code why not test ring-size on Stack Exchange first to see how it is received?
I prefer ring-size but also approved a decoys and possible-signers synonym based on community feedback.
I created a mixin synonym for ring-size. Currently both tags still exist. Now that we have reached broad consensus I think it is safe to move forward and merge the tags.