I first saw the C3X mod (which is the official name of Flintlock's mod) talked about by
tjs282 in a succession game thread
@tjs282 . After reading what he said and some others, I tried it, and thought I wasn't going to submit anymore hall of fame games. I say this, because In 2022, I might have submitted a game at some level or thought about doing so if some version of it had been legal at the time. Perhaps someone else might submit a valid entry to the HoF if it becomes legal which they would not do so if Flintlock's mod remains illegal in all versions.
So, with respect to why should it get allowed; because doing so might result in more entries. Also, I believe that longtime players would enjoy the bugfixes and it's conveniences, such as arrows in the trading screen to go back and forth between AIs, and stack bombardment. I haven't used a Scientific Great Leader for a real scientific golden age, but it just makes so much more sense for a spaceship HoF game to have a scinetific golden age as a meaningful option than mindlessly rushing Newton's University or needing to wait for a while to use it. Seeing an AI actually have a cavalry army also made for an exciting experience. Though, I just shot it down with an army I had, and though once I saw an AI with a second offensive army, I took care of that also, and they won't cash rush armies and likely won't figure out how to mass farm armies, nor think to atttack the player's army first instead of weaker targets (maybe).
2) On the other hand reasons why to disallow it include that it has so many different possible settings. Perhaps most conspicuous, the AIs behavior can get changed by changing it's artillery build ratio. They can move non-captured artillery type units out of cities and use them offensively in any case with Flintlock's mod. I did find that fun. Except, later once I played on Sid, and it seemed like the AIs were poorer. A few days later I started thinking that with the AIs building artillery type units more, they might end up more economic problems more generally due to them building artillery type units instead of more expensive units (attackers and defenders of similar eras tend to cost more than artillery type units). That could change thier research rate. Playing an 80% archipelago map on Emperor (or maybe it was Monarch), I saw some AI drop off I think 2 units, one an attacker and one a catapult. Then the attacker disappered ... something that happens even in classic C3C. But I could capture a catapult, instead of needing to kill one unit instead of two like in classic civ III. I think a few people, not just one person, who have used versions before the recent lowering of the default artillery build ratio commented how they would just never build any artillery after their first war. Though, there were many designers, and no doubt they had some disagreements on what the game should be like also, arguably, some of the changes stray too far from the designers's intent or what they thought civ III should be like.
Though, perhaps the designers intended variation when in doubt, so changes more fit with why it should be allowed than why it should not be allowed.
Also, allowing all the settings that Flintlock's mod has might lead to "analysis of paraylsis" for some people. Them spending all sorts of time experimenting with it's multiple possible settings, instead of finishing a game and submitting it.
3) If you allow some version of it, I predict that it will get used and some of us will recommend using it when considering starting up a new game. I don't think any of the 20k tables likely to change because of it. I could imagine that some of the diplomatic or spaceship tables could have their dates bested with a scientific golden age as a meaningful possibility, if people had an interest in playing for those games. I can imagine more histographic entries appearing on those tables (just barely), since the convenience features I think make it more likely that players would finish a game, since they would feel it less of a grind. I predict more submissions than not if you allow some version of it.
Vanilla, PTW, and Conquests all have their own markers, so we can distinguish those games on the tables, and I think Flintlock should also.
@Quintillus said something similar elsewhere. I will third this.
(And for me: to read and understand the competitive benefits of Flintock's Mod! 
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