Found some of these in jinja files:
</extension>
{%- else -%}
<extension name="blabla">
And also
{% if freeswitch_dispatcher -%}
<extension name="hold_unhold">
See the dashes ? Any idea what it's for ?
Answer
Turns out that + and - are there for whitespace control purpose.
You can manually disable the lstrip_blocks behavior by putting a plus sign (+) at the start of a block
[...]
You can also strip whitespace in templates by hand. If you put an minus sign (-) to the start or end of an block (for example a for tag), a comment or variable expression you can remove the whitespaces after or before that block