Deprecated in 8.2: https://www.php.net/manual/en/migration82.deprecated.php#migration82.deprecated.core.dollar-brace-interpolation
This is in the monitoring code we that we ship in the PHP production image, which powers the custom /metrics endpoint (offending line).
Since this happens outside of the MediaWiki serving path, this does not end up in said logs, and can only be seen in the php-fpm error log (example entry).
In any case, I'll put together a patch to fix this. Tagging serviceops since this is ours.