> These fixes will also be(come) available to the main OpenJDK distributions.
This isn't true! Oracle is under no obligation to mainline fixes from their JDK into OpenJDK.
That work is somewhat happening via IBM and Redhat, however, oracle is not doing it.
Further, new cuts of the OpenJDK are not happening for those old versions. You MUST get the fixes via a build of the OpenJDK that isn't the Oracle build of the OpenJDK (like adopt OpenJDK).
You can only depend on the "LTS" if the vender of your JDK supports LTS. There's no technically about it.
Exactly so officially you can indeed only rely on what your distribution promises you. In practice you may, with varying success, assume that if you are using a version that is LTS by one of the big distributions some fixes will land in mainline as well and therefore also in your distribution. Particularly for really critical fixes.
Of course, only officially rely on whatever agreement you have with your distribution/distributor.
Therefore, as a user of any JDK distribution you can depend a bit on the LTS versions, in addition that what your distribution promises.