Chrome only recently added HEVC video support (v107).
Plex then updated the hosted version of Plex Web (app.plex.tv) to also support HEVC video (v4.94.2).
The version of Plex Web bundled with Plex Media Server always lags the hosted version by several releases. When Plex updates it to 4.94.2 or later, it will support HEVC video (unknown when that will happen).
Safari has supported HEVC playback for quite some time. There may be some base level hardware and MacOS requirements.
Additional info on browser HEVC support: HEVC/H.265 video format | Can I use... Support tables for HTML5, CSS3, etc
