find cannot do on having space within string variable for -path or -iregex or etc

Don't put actions into a string variable that contains the data (see Bash FAQ 50). And always double-quote your variables when you use them

i='/home/demo.*/.config/File System/.*t$'
find ~ -iregex "$i"

If you want to include the -iregex only if $i is not empty then you can do something like this, which keeps it but replaces the regular expression with .* (i.e. anything)

find ~ -iregex "${i-.*}"

If you are using a shell such as bash (but not sh) that understands arrays you can use one to include the -iregex only if needed

IFS= read -r i

findRegex=()
[[ -n "$i" ]] && findRegex=('-iregex' "$i")

find ~ "${findRegex[@]}"

To see what's going on here you can modify find to echo find. Reading man bash and searching for Arrays and/or @ may also help.

Notice I've also changed your read i to IFS= read -r i. This stops the shell trying to parse your input data, which can break expressions containing multiple spaces (characters in $IFS to be precise) and other character combinations