Hi Gourav,
That's a validation that is (at least in my installation) set to Severity 'Warning', which you can ignore for now. It would be a good idea to adjust code over time to not use any deprecated methods, but I would be more concerned about validations with the Severity 'Error'.
If you go to the System tab > Audit group > Design Control > Stage Validation > Advanced, the ""Expression... uses the function ..., which is deprecated"" warning can be changed to 'Advice' severity or unchecked completely. It's probably best not to change this. I only bring it up in case it's a requirement for you guys before code promotion is approved.
Respectfully,
Dave
Dave Morris, 3Ci at Southern Company