Looking at the WordPress Sites we manage, none of them are using a "?keyword=" tag. All of them are using "/?s=" except for two sites which we changed as we were doing some predictive search (as you type, it finds related words).
With that said, there's very few of the pages that have "?s=" in the URL were a landing page, they were nearly always a search that was done in the search box on the page itself.
So I decided to bring up Analytics for one of the sites looking at 2010 to today.
On there, there have been 54,837 sessions with 29 sessions showing as a landing page starting with ?s=
10 of those sessions have "organic" listed as the medium the rest were referrals (essentially people linking to our results page to show all things related to "bert" for example.)
Of the 10 organics that showed, they were all from September 2019, which in the notes for Analytics it says was a revamp of our website changing themes and moving to Elementor for design. So I'm guessing that we had the search results pages indexed for a few days before they were deindexed again.
(side rant: I wish GA4 had the ability to add notes!)