[TeMPOraL]

Another idea that could reach a significant fraction of web developers: have the crawler check for potential use of deprecated features and alert the website owners through Google Search Console e-mail flow (using a separate message from regular GSC updates). The people who read GCP reports are likely to get worried and forward it to devs responsible for the site.

Another variant of this: surface such warnings on Google Analytics page/report. The easiest way to get software developers to do something is to convince sales&marketing that conversions will drop if they don't do it.