Ordinals are also allowed by "default behavior of Bitcoin Core", but they are filtered by some mining pools anyway.

data carrier: https://github.com/bitcoin/bitcoin/issues/27043
bare multisig: https://github.com/bitcoin/bitcoin/pull/28217

In general, if you make for example 1-of-20 multisig, then guess what: you need a single valid key, and the rest 19 keys can be used, just to push some data.

And in case of OP_RETURN, the 80-byte limit is no longer a problem, because there are pools accepting more than that. So, obviously, it is used as a faster way of spamming, because then, you can put your spam inside some output, instead of input (which is needed to put more spam, if you want to send a lot of short messages, because then, it may be cheaper than Taproot, if your data is between zero and a few hundred bytes).

Also, bare multisig can be used to slow down verification, and to increase the number of sigops in a given block. And both issues can be used, to get some advantage over another mining pool, which may be unaware of that, and include those transactions, and losing some time or fees by doing that.