What was the old default font prior to JetBrains Mono

Not sure... may be Consolas or Segoe UI

Default fonts used before 2020.1 version depending on the OS:

public final static String FALLBACK_FONT_FAMILY         = "Monospaced";
public final static String MAC_OS_DEFAULT_FONT_FAMILY   = "Menlo";
public final static String LINUX_DEFAULT_FONT_FAMILY    = "DejaVu Sans Mono";
public final static String WINDOWS_DEFAULT_FONT_FAMILY  = "Consolas";

I use PhpStorm on Ubuntu. Fonts "DejaVu Sans Mono" & "Monospaced" solve the problem:

Thanks to Elena Pogorelova

For me to get the bold properties back it was just to change the color scheme to "Classic Light" and setting the font size to 12 instead of 13.