Simple present or Simple past?

Hi Everyone,

The new website looks great! Congrats on the update.

I have a question about the following two sentences that I wrote up for my class last Thursday.

'I would have thought that it was mandatory to follow the new rules.'

and

'I would have thought that it is mandatory to follow the new rules.'

I know we can say both, but why?

I wrote the first sentence on the smart board, and a student asked me why we can't say 'is' instead of 'was'.

I told him you can say both, but I wasn't sure why.

Thanks

Miss slightly confused.