<targets> in commands allow for the + and . characters (- as well, but that's part of a UUID) and quoted values.
The following commands/values are correct:

/advancement grant ++++++ everything
/execute if entity ...........
/effect clear "spaces and more!"
/data get entity "1-0-0-0-1" (this acts as an entity UUID)

This is probably caused by just reading the next string (readString()) in Brigadier, which allows for both normal and quoted strings (readUnquotedString() and readStringUntil(char terminator)) where + and . are also allowed in isAllowedInUnquotedString().