If you are rooted with Magisk then you can install an alternate webview. As mentioned the Bromite webview is stalled.
Mulch webview (from DivestOS) is regularly updated and has been added to Open WebView module.
I use Mulch SystemWebView Overlay on my ROMs. It is a straight flashable zip but with alternate ways of installing if it does not work (like via adb). A Magisk module is also available.
I use Mulch webview on all ROMs including Oreo. I do not run OEM stock so no comments/ideas on that front.
Once the webview is installed and visible it can be updated regularly using your preferred F-Droid client with Divest OS repo added.
What ROM and what method did you use to install it?
On my ROMs/devices the zip from the link I posted never worked. Since all of my ROMs (custom, not OEM stock) are rooted I just manually placed the overlay apk where it needed to be (usually /vendor/overlay/ and permission set to 644) and then installed Mulch WebView. Reboot for good measure and it was available under Developer Options.
If not rooted then I figure the adb method should work.
The aforementioned Open WebView magisk module, which I've used in the past for Bromite, fails to install currently.
There was bromite but that seems to have died.
open webview has some similar webview replacements I think, but I havent gotten it to install yet so can't promise anything.
If I remember correctly GrapheneOS develops their own chromium fork called Vanadium, that also provides a webview. I don't know if it's installable on other ROMs tho