Retired corrections officers are indeed eligible for a $15,000 exclusion in Maryland on their pension under the conditions described above. However, rather than being reported on form 502SU (where most other subtractions are reported), this particular pension exclusion appears on line 10b on form 502. Note that this exclusion comes from Pension Computation Worksheet 13E on page 10 at https://forms.marylandtaxes.gov/18_forms/Resident_Booklet.pdf.
See the instructions for "10b. PENSION EXCLUSION FOR RETIRED CORRECTIONAL OFFICER, LAW ENFORCEMENT OFFICER, OR FIRE, RESCUE, OR EMERGENCY SERVICES PERSONNEL" on page 7 at the link above.
The answer at the link that Hal_Al referred to appeared to have been written before the actual instructions for this exclusion were written by the state (if the law was changed in July 2018, it obviously took time for the state department of revenue to redesign the forms and write the instructions and make them available to tax software companies like Intuit). In the event, how the state treated these pensions changed and the current instructions can be found at the link above and at https://taxes.marylandtaxes.gov/Individual_Taxes/Individual_Tax_Types/Income_Tax/Filing_Information/....
