https://www.androidauthority.com/esim-adapter-android-phone-3480601/
This eSIM adapter card lets you use cheap eSIM plans on any device with a SIM card slot, i.e. most Android phones.
https://jmp.chat/esim-adapter it's realer than you think and it works. Do you have a source to some documentation that says eSIM works only with the proprietary Google code?
Grapheneos people have said so repeatedly in their documentation. At least for the activation part. Once the eSIM is downloaded/installed, it's no longer needed i think.
Oh, wait... I didn't think about this and didn't know it was closed source, even though when I think about what google is doing lately it's no surprise.
I'm not the person you're asking, but I have some evidence to support the case that making it work without proprietary code is a problem. GrapheneOS, a privacy-oriented Android-based smartphone OS, write in their usage guide:
By default, GrapheneOS has always shipped with baseline support for eSIM, where users can use any eSIMs installed previously on the device. However, in order to manage and add eSIMs, proprietary Google functionality is needed. This is fully disabled by default.
eSIM support on GrapheneOS doesn't require any dependency on Google Play, and never shares data to Google Play even when installed.
Edit: The fact that they haven't implemented eSIM functionality without using the proprietary Google stuff, indicates to me that it's either impossible/blocked or simply too hard with practically nothing in return to have been prioritised (yet?)