Flagbearer ability is a weird one to dig up for an one-off design. It has certainly been taunted around as a keyword candidate. Ie.
"you must target a creature with [keywordname] if able".Making it just one card is weird in that it sort of makes any future ideas of a keyword more awkward the more of these there are around. As in, I think having that Protector and
Standard Bearer on the battlefield means opponent simply can't use single-creature-target spells altogether. Having the two ignored is fine, maybe even banned if deemed necessary, but the problem is if there would be multiple modern legal cards with similar abilities that together lock people out of casting most of their spells simply by being on the board.
Since currently only
Coalition Honor Guard and
Standard Bearer exist, it's relatively easy to deal with two nonmodern legal cards one way or another if a keyword was made. Technically you could have all the new creatures with the new keyword have the Flagbearer creature type for them to work together although that seems rather restrictive flavorwise. However, if you just start pumping out creatures with this sort of ability, then it becomes a mess quickly. It also seems like a waste of a design space considering how important keeping these uniform seems.
All of that being said, I appreciate the design sense here that such an effect, which can considerably restrict how opponents can cast their spells, would be on the side of the daybound, which transforms out of it when a player casts no spells on their turn. I would still rather have the effect be tied to some sort of a proposed keyword but I suppose in contextless custom design analysis it's like whatever.