Carpet installers are infamous for removing thresholds when Installing their new carpet because it didn't quite work out with their work and they never put them back. They don't know how however most of the expert flooring contractors know about this issue and have us ready to finish the threshold replacement and weathering.
The same thing applies to tile, wood or laminate flooring. They never put the thresholds back properly. Hopefully you have a great Installer that has warned you in advance that you may need someone like us to work with them.
There is a right way to do things when it comes to construction and unfortunately most Contractors just want to do their work first and that's wrong. Contact me and I will tell you when or even if you need to worry about this annoying issue.
When doing carpet the threshold needs to be in place and the carpet butts up to it and If you're having a plush thick carpet and pad then the threshold needs to be a higher model. If you're having wood or tile the rules are different. Call and ask me for free.
I go out to so many homes where the flooring guys have removed the threshold cause it didn't work with what they were doing and for the most they've been left high and dry to get it fixed because after al they've told you that "It wasn't their fault"
The really good flooring Contractors have their customers contact me ahead of time to come in and make the threshold right before they start work so that you're not left with a gap under your exterior doors for draughts and bugs to get in and on occasion I turn up after the Installation. It all depends on the floor type.