1. While true you don't need an HDMI 2.1 receiver for eARC, I think it would be premature to buy a receiver now without HDMI 2.1. If you already have an HDMI 2.0 receiver, just wait a year or so and get an HDMI 2.1 receiver then. If your receiver can be firmware upgraded to support eARC, then even better, but my two receivers can't be. I'm not complaining though, since they're both HDMI 2.0 models (Marantz and Denon) from 2015, yet both were upgraded in 2018 to support Dolby Vision. That's all I need for the time being.No No NO!
You are providing really bad information.
The AVR can support eARC without being HDMI 2.1. There are numerous 2017 and 2018 and 2019 HDMI 2.0 AVRs that have been firmware upgraded to support eARC.
Also, no streaming services currently support HD audio... upgrading to eARC won't magically make your source material HD.
Dolby Atmos from Netflix and Amazon streaming services is NOT HD AUDIO. It is low bit-rate Atmos that is sent over the digital dolby + 640kbps channel. "Real" Atmos is sent over True-HD. If you want the full experience via eARC you would need a game console, BD player, etc., with HD audio content that can be sent over eARC to your eARC capable AVR.
Dolby Vision support on receiver + Apple TV 4K for Atmos = Happiness.
No need for eARC for new equipment, and as mentioned, my TV (2018 LG C8 OLED) doesn't support eARC anyway.
2. Regardless if the Dolby Atmos is compressed, the bottom line is there is no standardized way to transmit even compressed Dolby Atmos over ARC. It can be done, but it's non-spec, and it usually wouldn't work. Therefore, if you really want Dolby Atmos, even the compressed version, you still need eARC... or else something like an Apple TV.
BTW, right now, the HDMI 2.1 endowed 2019 LG C9 OLED has problems with eARC anyway. Various issues are reported with various receivers. I don't know if it's just early adopter growing pains, or if it's because either the LG or the non-HDMI 2.1 but eARC capable receivers aren't 100% up to spec for this. I'm not surprised though, since none of those receivers are officially HDMI 2.1, so it wouldn't surprise me if some of those are out-of-spec.