We know that oral sex does carry risk for STD transmission. The risks vary depending on which infection we are discussing.
Some STDs are transmitted through skin-to-skin contact, including oral-genital and oral-anal contact. For example, herpes and syphilis are spread by contact with infectious sores or lesions, so oral sex could definitely spread these infections regardless of ejaculation. HPV can also be spread from genitals to mouth, or from mouth to mouth (http://cebp.aacrjournals.org/content/23/12/2959). Ejaculation during oral sex makes no difference with any of these infections.
Many people believe that gonorrhea and chlamydia can only be transmitted if ejaculation happens, but this is not true. The infections live in the epithelium (skin cells) of mucous membranes, like the cells lining the urethra (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC523569/ and https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4886739/). The bacteria are present on these cells, and can be transferred to the throat without ejaculate (cum) or even pre-ejaculate (pre-cum) being present. Interestingly, one study (https://sti.bmj.com/content/93/2/88) showed that gargling with Listerine reduced the amount of gonorrhea bacteria in the throat, but this study didn’t follow up to find out if this means fewer cases of gonorrhea were transmitted from throat to genitals.
Oral sex with or without swallowing semen both carry a very low risk of HIV infection – so low that we cannot calculate an accurate number (https://www.cdc.gov/hiv/risk/estimates/riskbehaviors.html). We do not have any studies that look specifically at the difference in risk without ejaculation.If you are concerned about the low risk of HIV transmission via oral sex, you could communicate this to partners and then not swallow any semen that is present. From a harm reduction perspective, this may be less risky, but we just don’t have any scientific proof to say for sure if swallowing makes a difference.