Custom field + twig solution:
It is also possible to use a combination of custom field in 'user' and then print in comments.html.twig
[edit to add version that does not use '|raw']
{{- content|without('links') -}} {# suppress links #}
{# add sig #}
<div class="usersig">
{% set usersig = comment.getOwner.YOUR_FIELDNAME.value %}
{% if usersig|length == 0 %}
{% set usersig = ' ' %}
{% endif %}
{% set sigbody = {
'#type': 'processed_text',
'#text': usersig,
'#format': comment.getOwner.YOUR_FIELDNAME.format,
} %}
{{sigbody}}
</div>
{# add previously suppressed links #}
{{content.links}}
[end edit]
Original method (probably not a good idea)...
{{ comment.getOwner.INSERT_CUSTOM_FIELDNAME.value|raw }}
The "|raw" is for formatted text field (renders the html as html). If your field is plain text omit that.
The advantage of this approach is that it is lighter and does not depend on a module that is not ready for production use. Disadvantage: the module appears to offer some more features.
On the safety of the |raw switch - One method is to use the Twig Tweak module's check_markup feature. That and other options discussed here: How to print text (formatted, long) in twig without raw filter?