Couple of quick notes on fixing templates that used the {structure_entries} tag under EE2 when you've upgraded to EE3/EE4/EE5/+
I've inerited a number of EE2 sites that we want to upgrade to EE5, they were built using {structure_entries} but that plugin hasn't been updated since EE2 and no-longer works with newer expression engine versions...
So, generally structure_entries is used for building a sub-nav, listing page links from a branch of the nav tree. I assume this functionality didn't originally exist in Structure, hence someone created structure_entries, but (for my use-case at least) you can achieve the same functionality in Structure itself these days with a pretty simple code translation, meaning you can ditch {structure_entries} completely.
I found code like this in a template to build a sub-nav:
{exp:structure_entries depth="1" parent="/{segment_1}/{segment_2}" }
...
<a href="{page_uri}">
{title}
</a>
...
{if "{channel} == "article"}{an_article_field_name}{/if}
...
{/exp:structure_entries}
And translated it into the following code that works under Structure itself:
{exp:structure:nav_advanced start_from="/{segment_1}/{segment_2}" max_depth="1" parse="inward"}
...
<a href="{root:page_url}">
{root:title}
</a>
...
{if "{root:channel_short_name}" == "article"}{root:an_article_field_name}{/if}
...
{/exp:structure:nav_advanced}
(Root in this instance starts from the tree branch you're initially starting from, not the Root of the structure tree (if your subnav was multi-level then for the child items you'd use child: instead of root: etc)