That's an issue primarily with the hyperscalers, who're refusing DANE. Anything outside of Gmail, Outlook and the likes, you're much more likely to see DANE than MTA-STS.
(Most open source SMTP stacks don't integrate an HTTP client and are completely unable to do MTA-STS without hacks, patches or other fragile bolt-on bits.)
(Most open source SMTP stacks don't integrate an HTTP client and are completely unable to do MTA-STS without hacks, patches or other fragile bolt-on bits.)