There was a mention of a browser engine update done in the release notes of the latest SFOS 5.0 update for the C2, so I guess the ESR91 browser is now being maintained as a part of SFOS 5.0.
Which of course means that those of us on non-C2 devices will have to wait for SFOS 5.0 to land on rest of the devices. In the most recent community newsletter it was communicated that their target is to release version 5.0 for all supported devices at the same time the next OTA update for C2 is released - no planned date was announced, however.