Quote from up2ng >>

I remember some old threads on this topic where it was argued that if you were a creature that normally had a 5-foot reach, and you were wielding a weapon that causes your reach to increase to 10 feet when attacking with that weapon . . . that if an enemy was within 5 feet and exited your 5-foot radius that you could choose to make an opportunity attack by making an Unarmed Strike by, say, kicking the enemy for example.  Of course, if you chose to do that you would be using up your Reaction to do that and so if that enemy immediately proceeded to exit your 10-foot radius then you would no longer have a Reaction with which to make an opportunity attack with your weapon.

Not sure if any of that would still be a thing in the 2024 rules without looking into it further.

None of that is any different in the 2024 rules.

Right!

There's a related entry in the SAC:

How does a reach weapon work with opportunity attacks?

An opportunity attack is normally triggered when a creature you can see moves beyond your reach. If you want to make an opportunity attack with a reach weapon, such as a glaive or a halberd, you can do so when a creature leaves the reach you have with that weapon. For example, if you’re wielding a halberd, a creature that is right next to you could move 5 feet away without triggering an opportunity attack. If that creature tries to move an additional 5 feet—beyond your 10-foot reach—the creature then triggers an opportunity attack.

And from the Dev (link to sageadvice.eu):

@jd2319 What if I'm dual-wielding a dagger and a whip, can I use the dagger when someone leaves my 5' but stays w/in 10'?
@JeremyECrawford If you want to make an opportunity attack with X, the attack is triggered when a foe leaves the reach of X. X = weapon you want to use
@thethain Awesome! So does that mean you can always make a 5ft unarmed Opportunity Attack, IE if holding a two handed weapon?
@JeremyECrawford Unless a feature tells you otherwise, you have a 5 ft. reach with your unarmed strikes, and you can make opportunity attacks with them.

Anticipating your question: Regarding the 3.5 Edition "Attacks of Opportunity" (AoO) rules.

Sometimes, however, a combatant in a melee lets her guard down. In this case, combatants near her can take advantage of her lapse in defense to attack her for free. These free attacks are called attacks of opportunity (see the diagram on the next page).
Threatened Squares: You threaten all squares into which you can make a melee attack, even when it is not your action. Generally, that means everything in all squares adjacent to your space (including diagonally). An enemy that takes certain actions while in a threatened square provokes an attack of opportunity from you. If you’re unarmed, you don’t normally threaten any squares and thus can’t make attacks of opportunity (but see Unarmed Attacks, page 139).

……

Provoking an Attack of Opportunity: Two kinds of actions can provoke attacks of opportunity: moving out of a threatened square and performing an action within a threatened square.
Moving: Moving out of a threatened square usually provokes an attack of opportunity from the threatening opponent. There are two common methods of avoiding such an attack—the 5-foot-step (see page 144) and the withdraw action (see page 143).
Performing a Distracting Act: Some actions, when performed in a threatened square, provoke attacks of opportunity as you divert your attention from the battle. Casting a spell and attacking with a ranged weapon, for example, are distracting actions. Table 8–2: Actions in Combat notes many of the actions that provoke attacks of opportunity.
Remember that even actions that normally provoke attacks of opportunity may have exceptions to this rule. For instance, a character with the Improved Unarmed Strike feat doesn’t incur an attack of opportunity for making an unarmed attack.

As cited in the text, this rule clarifies "all scenarios where you might provoke an AoO by not engaging in melee combat during battle," and further provides a comprehensive table outlining specific triggering conditions.

I'm uncertain if this reference falls under "long quotes on game content not covered by the SRD." If my post violates community guidelines, kindly notify a moderator to remove it. Thank you for your understanding!