Every time I look at a which carrier to book for a flight I wish there was a way to go to a single resource (without visiting each carrier's site) to know at a glance which airline gives me the most lenient restrictions, especially when they are at the same price point. Is there such a resource that is updated frequently as new rules come out?
There are multiple possibilities already existing to have a more or less refined idea of the baggage restrictions. It depends if you want the detailed restrictions on bag sizes or just the cost of a checked bag.
Expedia shows the price of carry-on and checked bags on their flight details, like in the screenshot below.
Seatguru shows, for each airline, the detailed baggage rules, this one is for Air France. You have to pick your airline and then click the Baggage option in the left pane. The page shows rather detailed information. At the bottom of that page, you have a link to the airline's baggage rules page, for even more details (and if the seatguru page is not up-to-date).
EDIT: As @verve commented, do not expect Seatguru to be up-to-date. You should probably better follow the links to the airlines' page.
Finally Skyscanner has a list of links to the airline pages regarding the baggage rules. These are the most accurate, since we do not know how often the two other update their websites.
These are maybe not exactly what you expected and require a few clicks, but it is relatively easy to access, you can bookmark the skyscanner list of links for example. And probably more flight search engines have integrated these, the list is not exhaustive.