Obviously this is an exaggeration. As long as they are behaving respectfully, a toddler’s feet can touch the floor or kneeler at Mass. I will let them stand in the pew so they can see. I have even left them to sulk quietly sprawled on the floor under the pew during Mass when that was the least disruptive option available to me. But what I try to avoid is rewarding disruptive behavior.
If you are disruptive, you must leave Mass, but leaving Mass will not be more fun than attending Mass. There isn’t running or exploring in the back of church. If you are quiet, you are held in arms at the back of church. If you are loud, you are held in arms outside of church. I recall one Friday evening stations where I spent the entire time in the hall outside of church holding a screaming toddler. It was very Lenten.
My children have since broken my body including my back, so my husband is usually the enforcer on this front. But occasionally, it falls to me, and we sit on the stairs, and I hold them firmly in my lap.
There is typically a little more quiet freedom through age three in the pew (sideways pacing on the kneeler, switching laps, flipping through hymnals or missals, etc...), and eventually they seem to figure it out that being quiet in the pew is more fun than being loud and then removed without the option of escape.
We found early on that as soon as their feet touched the floor in the back of church, it delivered a thrill of hope and turned the consequence for bad behavior into a potential reward, so this rule was one of the first we came up with as newbie parents, and it has truly worked well with all our children.