Tools like (for MediaTek) or Qualcomm Tool can sometimes bypass SIM requirement by reading/writing to nvdata partition directly.
Unlocking a bootloader without a SIM card depends heavily on your device manufacturer. While many devices like and older Samsungs allow this with just a Wi-Fi connection, others like Xiaomi have strict requirements that typically demand a SIM card for account binding. Manufacturer-Specific Requirements The ability to unlock without a SIM card varies: how to unlock bootloader without sim card