I just lost my internship certificate, and don't think I may get a new one. Do firms ask them before giving an internship for past experience proof ? I know and can explain my work done in past internship

If you have record of the work on mail then those works as well

But is it a requisite ? I do have conversations on WhatsApp as it was a boutique.

See usually law firms don't ask for your certificates unless they find something suspicious.

Preferably, if you have acceptance of your internship applications on mail + if u can provide contacts of people working in the firm who can confirm about your internship then that works!