That's why often startups can't do business with big companies that hold to rules like that.

Would Smyte's rating have cut it for people?

Your supplier getting acquired is also a well known risk that you cover by reading/negotiating the contract.

The problem is, if all suppliers use the same contract clauses you may not have a choice.

How do you check the credit rating of a startup?

and how would their credit rating be affected if they pay all their bills but just reduce the number of bills gradually over time?

One way that's sometimes done is that you require them to submit an independent respectable audit of their financials, or in case of confidentiality issues, not submit the actual financials but a statement from a respectable auditor that they have verified the suppliers financials and their cashflow/"runway"/etc satisfies the criteria that you required.