There is a brand of ale called Farmers Ale. Should there be an apostrophe on farmers? Or is farmers acting as an adjective like "sports" cars?
The New Fowler's Modern English Usage (Revised by R. Burchfield 1998) has the following extract in its section on apostrophes:
Relinquishment of the apostrophe
Since about 1900, many business firms, institutions, and journals have abandoned apostrophes in their titles, e.g. Barclays Bank, Citizens Advice Bureau, Diners Club, Farmers Weekly, Harrods, Mothers Pride Bread , Teachers Training College. (It can be argued that in some of these the word ending in -s is a plural word used attributively.) ... This trend towards the dropping of the apostrophe in such names and titles seems certain to continue.
The bolded text above is relevant in answer to your questions.
You can find the entire text of Burchfield's book at the Internet Archive.
The construction normally uses an apostrophe as in the following two examples.
Green Book Standards
If Soldiers’ duties prevent them from conducting PRT during these hours, commanders will establish PRT at an alternate time during the duty day
Women’s rights are human rights!
Nevertheless, your example is a trade name rather than an adjectivally qualified ale. As such it does not explicitly refer to "Farmers' ale" as an ale contextually specific to farmers. The trade product "Farmers Ale" may be drunk by anyone.