YES, Flatee is legal in Singapore!
Most license plates are slot-in between the common plastic frame system which are easily detachable in a quick 2 seconds.
There is no safer, better and more reliable affixing system available in Singapore.
What you will anticipate from Vehicle Inspection Centers?
In actual fact, Inspectors at VICOM or STA do not have proper guidelines on how license plates must be affixed, therefore inspection approvals are based on individual assumptions, resulting in inconsistency between inspectors and inspection centres. (this fact has been disclosed by LTA)