tl;dr the crawlers do not respect robots.txt or the user agent anymore, but you can drop big bucks on the enterprise HA offering to stop them through other means.
I mean except known good actors.
I guess known actors would need a verifiable signature
https://developers.google.com/search/docs/crawling-indexing/...
https://www.bing.com/webmasters/help/how-to-verify-bingbot-3...
"Verifiable signature"? That's a dangerous road to go down, and Google actually wanted to do it (Web Integrity API). Nobody supported them and they backed out.
With salaries though finding an externally managed solution might be cheaper.