What are the consequences of leaving the US while my I-539 application is pending?

If you travel while the change of status is pending it is deemed abandoned. However, a non-frivolous application to change status may toll any unlawful presence issues. If you have a valid F-1 visa and your SEVIS record has not been terminated and you have a valid I-20, you may be eligible for readmission. The decision will be made by the inspecting CBP officer.

You need to notify USCIS of your departure and withdraw pending B-2. You can contact the school to see if the I-20 is still active in the SEVIS system to use for reentry.

If you timely filed COS application and left during pendency of the application, you should not have unlawful presence. COS application should be denied by USCIS due to abandonment.

Timely COS filing that is later abandoned generally does not result in unlawful presence during the COS pendency. It is always advisable to file a Withdrawal letter to protect your record. F-1 status, where admission is D/S, is treated differently than other valid admission periods. I would start by speaking with your DSO.

If you filed an I-539 from F-1 to B-2 in a timely fashion, then you would likely not have Out Of Status to worry about. But leaving the US prior to the adjudication of the I-539 does terminate the I-539. If the F-1 visa foil in your passport is still valid (i.e. it has not reached its expiry date) then it can also be used for a new I-20 study arrangement with your academic institution, even if you have previously been unable to maintain a full course of studies in a prior stay. It is advisable to consult with an immigration attorney in order to confirm that each phase of your prior nonimmigrant stay was resolved lawfully.