Is the S4-5a sign is still used?

No.

The S4-5a, Reduced School Speed Limit Ahead Sign text only sign is not allowed in New York State. The embedded speed limit sign version (S4-5) is the sign that shall be used. In addition, the color shall be the newer fluorescent yellow-green.

Here is the text from the NYS Supplement.

Section 7B.16 Reduced School Speed Limit Ahead Sign (S4-5, S4-5a)

INSERT the following paragraph:

Standard:

The S4-5a sign shall not be used in New York.

Also, the text only advanced speed limit sign (W3-5a) is also not allowed. The embedded speed limit sign version (W3-5) is the sign that shall be used.

Resources

MUTCD info update - NYS LTAP Center - Cornell Local Roads Program