Any crawl space should be either well ventilated, conditioned/heated, or completely sealed off (not as common).
There have been mold, wood rot, and other problems in older homes that have crawl spaces without good ventilation. The severity depends on the local climate, with less problems for older homes in drier areas without floor insulation or tight construction. The space had some ventilation and was somewhat conditioned/heated due to the lack of insulation without a vapor barrier facing at the floor framing.
BUT, take that same older home and seal up gaps and add modern insulation with a vapor barrier (e.g. tar layer on the back side of modern paper faced batts) … a common thing homeowners do to “fix” cold floors above crawl spaces … and you have a recipe for problems, particularly for older dirt floor crawl spaces.
In one crawl space in an older home without adequate ventilation where the homeowner added faced batts insulation between the floor joist bays, my awl went completely through the bottom 2" of the wood framing due to stagnant unheated conditions created by adding the insulation … :shock: