Chances are: a titled player would keep it well hidden if they made an alternate account (with the exception of a speedrun account as @KeSetoKaiba mentioned). It might be speculation to assume that it is regular practice among titled players.
Perhaps it is like a myth that people would like to believe.
True for some of it (mostly the part I said 
), but assuming it is "regular practice among titled players", let us realize how insignificant that is too we "normal players" even still!
Since GMs and IMs combined make up less than half of 1% of all chess players, for simplicity, let us just include NM, FM and all other official titles into the top half of 1% as well. This would translate to every 200 random chess players you encounter, 1 player would be a titled player (this seems relatively accurate online as well as OTB, given the global rating pool).
Say roughly 100 "true" titled players are on chess.com at the time you seek a game yourself and assume the outrageous figure of titled players indulging in an average of 3 accounts; so we can divide 99 (close enough to 100 and divisible by 3) by 3 and we end up with 33 "primary" accounts.
Note that this would be the number of titled players online, but their higher ratings wouldn't even pair you against them unless your rating was similarly high on its own. 33 primary titled accounts would give 66 "alternate" accounts in this fictional scenario...and chess.com literally has millions of members. See how insignificant 66 accounts would be? Furthermore, the ones who would encounter them would likely be 2000+ rating even if the titled player didn't work on their rating much.
"Normal chess players" would be unaffected despite this scenario being very generous in the conspiracy-similar thought that titled players indulge in alternate accounts.
I instead trust that most titled players are straight-forward about their multiple accounts with chess.com staff. Furthermore, there is little incentive for them to hide accounts. chess.com grants titled players a second account if asked permission for repertoire secrecy anyway; asking for a 3rd, 4th, 5th account etc. just seems like more story-telling than anything factual. Why violate chess.com tos and risk losing access to all of your accounts, including the primary one, when you would likely get permission if asked for those accounts. I believe most are straight-forward with multiple accounts they do open (for things like speedruns or repertoire secrecy) greatly because there is really no up-side I see for them to hide it.
Does anyone know what percentage of new players are actually titled players with alt accounts? I had a conversation with someone who stated that titled players regularly make alt accounts to practice, then they abandon or close the accounts.
Is this a regular practice?
This almost never happens. In fact, lower rated players are much more likely to close accounts to re-open others for various reasons. What titled players sometimes do is create a separate "speedrun account", but chess.com staff gives them permission to do this (player attempting speedrun contacts chess.com staff before they attempt this to get permission and then they play once the alternate account is approved). However, the speedrun account "refunds" all of the rating points they "stole" from their opponents after the speedrun attempt is completed; this way, no one really loses. Simply "smurfing" with a second account is not only against chess.com terms of service (tos), but also wouldn't refund the points of the opponents who lost to your under-rated winning.
Another way to illustrate just how RARE these titled player alternative accounts are is just the basic math behind it. Do you fully realize just how uncommon titled chess players are? GMs and IMs COMBINED makes up less than half of 1% of all chess players! Even if most titled players had a second alternative account (and not all titled players do), there are so few titled players around that it wouldn't really impact chess.com members count significantly at all!