What kind of compatibility do you mean? WinSCP does not really talk to PuTTY, so it does not need to be compatible anyhow.
The only problem can be the new PuTTY key format (PPK3) introduced in PuTTY 0.75. So if you generate a new key with the default settings in PuTTYgen 0.75, you won't be able to use it directly in WinSCP. Though, there won't be any problem, if you use the new key via Pageant. And no problem whatsoever with the existing keys (in the old PPK2 format).
But even the new PuTTYgen can generate (or convert) the key in the old format.
Go to Key > Parameters for saving key ley files > PPK file version > 2
Having that said, we will definitely upgrade WinSCP to PuTTY 0.75 asap. Possibly even in WinSCP 5.18.5 – Depending on complexity of changes in PuTTY 0.75.
Edit: WinSCP 5.19 with support for PPK3 format has been released already.