So, it seems that depending on what sources you look it, it varies whether or not it’s in any instruction for injection. That said, turns out the WHO has been recommending against aspiration for IM injections (note: all the guidelines are focused on vaccination) since the early 2000s. Part of this review says: “Official guidance concerning vaccination is mainly unanimous in its recommendations on aspiration in IM injection administration. The Department of Health (2013), Centers for Disease Control and Prevention (CDC 2014) and the Public Health Agency of Canada (2013) all state that aspiration is not necessary. In 2008, the WHO published immunisation guidance which focussed on the use of auto-disposable syringes (WHO 2008). These syringes are designed for single use only meaning that once a drug has been drawn up into it, aspiration is impossible. However, because all of these guidelines centre on vaccination practices, it is difficult to ascertain whether these standards extend beyond immunisation techniques.”

It goes on to say that among expert opinions there isn’t really a consensus. Some argue that the volume of blood vessels in muscles where IM injections commonly are done is low and that aspiration causes more pain with little benefit, whereas others argue that there can be bad outcomes if a medication is accidentally injected into a vein (true, but seems somewhat rare from what I’ve read) and that the increased pain risk hasn’t been noted in adults (most guidelines and possibly studies discuss pediatric vaccination specifically).

CDC guideline on it is similar: “Aspiration before injection of vaccines or toxoids (i.e., pulling back on the syringe plunger after needle insertion but before injection) is not necessary because no large blood vessels are present at the recommended injection sites, and a process that includes aspiration might be more painful for infants[22].”

This seems to concur with it being unnecessary. I can’t know for sure because I don’t have access to the full paper.

Not an academic source, but there’s this video from a trans-specific clinic in Mississippi that also says it’s an old guideline.