Everyone is free to use whatever software they want, whether this is Bitcoin Core 30.0 (with or without touching this default setting), an older version of Bitcoin Core, Bitcoin Knots, Libre Relay, or anything else. In this sense Bitcoin users are, in a very real way, sovereign.
Either way, Bitcoin does not operate like a democracy. Because every node typically relays transactions to multiple others, if even a relatively small minority of users choose to run Bitcoin Core 30.0 (or Libre Relay or something similar), larger OP_RETURNs should in fact propagate rather freely. This probably can’t be stopped completely, but assuming Bitcoin Knots proponents want to at least meaningfully stifle this, they’ll need to convince some supermajority of node operators (perhaps 95% or more) to join them in their filtering efforts.