Each major version of Bootstrap will receive at least six months of support after it is retired, followed by six months of critical bug fixes and security updates.
This official announcement from the Bootstrap team is important as website maintenance and update planning will be more projectable for developers.
https://blog.getbootstrap.com/2019/07/24/lts-plan
According to this blog post, Bootstrap v3 will not receive security updates anymore and that’s why the Bootstrap 3 download option has been marked as deprecated and will be removed shortly. Once Bootstrap v5 is published, this new version will become our default Starter Theme and Bootstrap v4 will become an optional download.