I did a search and Body Battery is available on cheaper models like the Garmin Vivosmart 4 ($129 USD). I put into DuckDuckGo and verified:
"Garmin" AND "vivosmart" AND "body battery"
Link to Garmin Vivosmart 4: https://buy.garmin.com/en-US/US/p/605739
Proof: https://www8.garmin.com/manuals/webhelp/vivosmart4/EN-US/GUI...
I won't get a non-replaceable band one though, I have a tendency to sweat them to death :/.
https://support.garmin.com/en-US/?faq=VOFJAsiXut9K19k1qEn5W5
By coincidence I was looking to buy a Garmin smart watch!
BTW I never had covid but if my body battery gets below 20, I also feel like shit. That´s the point I guess.
You may not want Google (e.g. YouTube) serving you ads when it knows you are in a very low body battery state, for example.