Strongbox supports versions 1 and 2 of the KeePass format, that is, both the KDB and KDBX formats.
It supports all versions of the KeePass 2 (KDBX) formats (currently 3.1, 4.0 and 4.1).
Because KeePass uses a pluggable architecture some KeePass files using special plugins may not be supported by Strongbox (we don't know of any at the moment, but in principle this is possible). If you use any algorithms outside of the standard range of AES, Salsa20, Argon2d/2i, TwoFish and ChaCha20 you may find Strongbox is not able to open the file.