Therefore, as a user of any JDK distribution you can depend a bit on the LTS versions, in addition that what your distribution promises.
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.
Of course, only officially rely on whatever agreement you have with your distribution/distributor.
LTS is strictly a JDK vendor concept. (AdoptOpenJDK is a JDK vendor). How long a version is supported depends entirely on the vendor.
A lot of vendors have adopted the oracle LTS strategy, some offering even longer support. However, it's vendor specific.
Debian is, themselves, a JDK vendor as they are building OpenJDK themselves. So what LTS means for debian is entirely up to what Debian is willing to do to support it.
LTS is strictly a JDK/JRE vendor concept. Oracle's distributions have 11 as an LTS and will have 17 as an LTS. That, however, only matters if you are paying oracle for support.