My recollection of the "old days" may be a bit hazy, but I think comma delimited parameters were a work around for frameworks that did not support multiple values (or users not knowing how to handle it)
Example of a "correct" url
?value=A&value=B&value=C
Complete frameworks would have a method that returned the values as a list. Some like PHP required ugly work arounds where you had to name the parameter using the array syntax: value[]=A&value[]=B&value[]=C
Even if the framework supported multi-values, many preferred the shorter version: value=A,B,C and split the values in code instead
Example of a "correct" url
?value=A&value=B&value=C
Complete frameworks would have a method that returned the values as a list. Some like PHP required ugly work arounds where you had to name the parameter using the array syntax: value[]=A&value[]=B&value[]=C
Even if the framework supported multi-values, many preferred the shorter version: value=A,B,C and split the values in code instead