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
"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.
https://developers.google.com/search/docs/crawling-indexing/...
https://www.bing.com/webmasters/help/how-to-verify-bingbot-3...
With salaries though finding an externally managed solution might be cheaper.