Keep a local copy of the lease, not just the one on the platform - servers can shut down, accounts get deleted, and you'll need the document in 5 years if something goes wrong.
It's true that it's legal (the others are right about that), but the thing is really in the practical management. A platform is cool during signing, but after that you need to be able to access the file easily, download it as a PDF, print it if needed. Some landlords use kind of sketchy sites that lock down the document or ask for a login every time. Check before you sign that you can download your lease directly, without having to stay registered on the platform.
Also, ask them to email it to you once it's signed. It's not a crazy request and every landlord should accept it. You'll have two copies - one from the platform and one on your computer - and it's way more reassuring if you need to prove something later on.