For me, it depends on the kind of fanfic it is.
One-shots and drabbles generally are canon-character centric. This does not exclude them from having OCs completely, but I usually use them only if there is a need for them. As in, we know a person must have existed but we don't know anything about them, or for the sake of the one-shot must assume there has been such a character even if canon's logic alone does not necessarily dictate that. With that I mean, logic dictates that if we have a canon character that has been born, they must have (had) parents, even if canon does not tell us anything about them. Or that if character is a teacher, they must have had students.
On the other hand, if I want to write a one-shot about a canon-character angsting about, say, the death of their pet rabbit, and remembering how they bought it at a pet shop, how they used to pet it, etc. the one-shot's logic dictates there must have been an employee at that pet shop that said character met. Canon's logic does not dictate such a thing unless canon explicitly tells us that a. canon character had/has a pet rabbit and b. character bought that rabbit themselves/was there when it was bought.
Multi-chapter story depends on the time-period and the plot. If it's set in pre-story times, chances are we know the names of a few people from then and the personality of even fewer. Which means OCs are basically unavoidable in most cases. Same if it's set after the story's end, or in un(der)developed/"glossed-over" parts of the story, which happens when there are time-jumps, etc.
Otherwise, it comes down to the plot. The further you deviate from canon, the more likely it is that there is need for OCs. In such cases, the OCs are in my case never the main-character of the story, but they can play an important role. Say, a character turns down a job they accepted in canon and did something else, that means one will have to deal with different characters. Some may be canon (especially if the different job is also established in canon, possibly even with the character debating which of the two jobs to choose, but just picked the other one in one's fanfic than in canon) but others will not be, especially if it's a job where a character has to interact with not just colleagues and their boss, but customers as well. There are thousands of examples like that possible, and that's when still dealing with fanfiction that's mostly character-centric and where the OCs' existence is dictated by in-story logic. (Can't be born without parents, can't teach without students, can't sell clothes without customers, etc.)
Sometimes, OCs are necessary not because the story's logic dictates they should exist, but because you need a character to fill a role and all of the existing canon characters are for various reasons unable to fill that role. (Couple of possible reasons: wrong personality, wrong morals, wrong personal circumstances, wrong gender, wrong age, wrong appearance, race, religion, culture, nationality, etc. Yes, several of these may seem shallow or even prejudiced, but they can nonetheless be valid reasons why specific characters cannot fit a role. If for whatever reason you need a neo-nazi, that would exclude people of jewish faith, people of colour, etc. If you need a teacher at a male-only school, that would exclude females, newborns, children, teenagers, animals, aliens, prisoners, etc.)
At that point, you have a limited number of choices - slightly change a canon character (preferably while giving enough in-story reasoning that their behaviour makes sense, even if it would be OOC for their canon counterpart to do such a thing), bend a canon character so out of shape that it's an OC-pretending-to-be-canon, or add in an OC. Depending on the reasoning why certain characters won't fit, the former may not even be an option. The second option is hard to do in a way that won't infuriate anyone who has a passing familiarity with the canon character.
TL;DR: OCs are okay where OCs are for various reasons either needed or preferable to other options.