According to the https://github.com/nextcloud/server/wiki/Maintenance-and-Release-Schedule, NC 11 support ends with the release of NC 13. The developers gave an additional 2-3 month overlap, so you have enough time to plan and test your upgrade to NC 12.
Nextcloud 12 was released a coupled a months ago, provides new features and should be pretty stable by now:
If you really must have longer support, enterprise support guarantees a maintenance life cycle between 3 (standard) and 10 years (premium).