Does v1 still get security patches?

[cenkalti]

Is it safe to keep using Flask v1 and not upgrade to v2?

[ThiefMaster]

It's general not safe to use outdated/unmaintained software versions.

If we found a really critical issue we might at least consider backporting it, but it is not safe to expect or assume so --> You should absolutely update.

Providing free "LTS-style" support for old versions is not something particularly sustainable for open source projects.