The Vienna State Opera is releasing one opera a day, up for 72 hours (subtitles in multiple languages) https://www.staatsoperlive.com/live

The Metropolitan Opera (NYC) is releasing one opera a day, up for 23 hours, has English subtitles. https://www.metopera.org/user-information/nightly-met-opera-...

https://operawire.com/ has a complete list of opera companies doing free streaming.

Wikipedia has details and synopsis for many operas.