Short answer
Yes, additionality is relevant in cases where a consequential GHG reduction or impact claim is being made or implied by a company or other consumer. However, neither the certification nor issuance process for RECs and GOs involves any kind of meaningful additionality assessment.
Long explanation
RECs and GOs are sometimes explicitly or implicitly claimed as serving the same function as GHG emission offset credits (consequential accounting impact claim). However, there is no evidence of additionality in voluntary REC and GO markets – they have not empirically induced greater renewable energy generation nor is the issuance of a REC or GO subject to any kind of meaningful additionality assessment. Instead, RECs and GOs are issued for generation arising from any qualifying resource, regardless of whether that resource would have been built and/or operated in the absence of REC or GO markets. Offsetting claims associated with RECs and GOs are therefore invalid.
In some instances, quasi-consequential arguments have been used to justify the use of RECs and GOs in attributional accounting (i.e., corporate GHG emission inventories). However, if you are preparing a corporate GHG inventory, the question of additionality should not enter the discussion. Any claim of additionality that is used to justify an estimation method or assumption for a corporate inventory is categorically flawed.[1]
[1] Brander, M. (2021). The most important GHG accounting concept you have never heard of: the attributional-consequential distinction. Seattle, WA. Greenhouse Gas Management Institute, April 2021. https://ghginstitute.org/wp-content/uploads/2021/04/Consequential-and-Attributional-Accounting-April-2021.pdf