Nintendo's not particularly consistent when adding new games to their Switch Online service. Similar to Microsoft's Games With Gold and Sony's PlayStation Plus but for a lower cost, new additions are frequently added for subscribers, though this only covers NES and SNES games.
However, if new reports are to be believed, that could change next month. Perhaps in conjunction with Switch Online's 3rd anniversary, NateDrake of the latest "Nate the Hate" podcast advises that Game Boy and Game Boy Color titles could soon arrive.
Nintendo Reportedly Adding Game Boy Titles To Switch Online Service
It's been known for sometime that the Switch houses four different emulators, two of which - Kachikachi and Canoe - cover the NES and SNES, respectively. During this podcast, Nate and co-host MVG stated they believe no.3, Hiyoko, covers both Game Boy libraries and that Nintendo will launch this in the coming weeks. However, no.4 (Count) remains an unknown.
Elaborating on this further, Nintendo Life's reportedly confirmed with their own sources that Game Boy/Color games are joining Switch Online "very soon", though Game Boy Advance games don't appear to be included. For now, that remains officially unconfirmed, so take this with a grain of salt, but if there's any truth to these rumours, we'll keep you updated.
Source: Nintendo Life