At the thread Is it correct to say 'I write children books' ... not using the possessive case? one finds
There is a move away from the use of the apostrophe in the
'adjectival' (rather than the 'true possessive') sense you mention
here. However, the tendency is to just drop the apostrophe rather than
switch to an unusual-sounding singular modifying noun:
Working Mens Club
Mens Clothing Department.
It's interesting that the aid to driving invented by Percy Shaw of Halifax (UK) was called the cat's eye, plural cat's eyes, but that nowadays the modern forms cats eye / cats eyes are usually used.
Correspondingly, the unapostrophised travellers cheques is the more modern usage, as can be found say at Editex. Though many dictionaries still give the older traveller's cheques. (This may seem the less logical possessive choice, meaning 'cheques for travellers' as it does, but corresponds to 'cheques for the average traveller'.)
However, customers' requirements is by no means so fossilised (travellers cheque/s has achieved compound noun status). But the modern trend would require the attributive noun usage customer requirements (using the singular-form attributive noun, as is the usual case).
This is supported by these Google Ngrams (which also show the near-equality in the use of customer's requirements and customers' requirements ... though they fail to distinguish the generic and the non-generic usages of both customer's requirements and customers' requirements).