Steps to reproduce the issue

use com_finder and check error logs

[Sat Dec 21 13:51:19.834493 2019] [fcgid:warn] [pid 31528:tid 708] [client ::1:52503] mod_fcgid: stderr: PHP Deprecated:  Array and string offset access syntax with curly braces is deprecated in C:\\htdocs\\joomla-cms\\libraries\\vendor\\wamania\\php-stemmer\\src\\Utf8.php on line 246, referer: http://localhost/joomla-cms/administrator/index.php?option=com_finder&view=indexer&tmpl=component
[Sat Dec 21 13:51:19.834993 2019] [fcgid:warn] [pid 31528:tid 708] [client ::1:52503] mod_fcgid: stderr: PHP Deprecated:  Array and string offset access syntax with curly braces is deprecated in C:\\htdocs\\joomla-cms\\libraries\\vendor\\wamania\\php-stemmer\\src\\Utf8.php on line 262, referer: http://localhost/joomla-cms/administrator/index.php?option=com_finder&view=indexer&tmpl=component
[Sat Dec 21 13:51:19.834993 2019] [fcgid:warn] [pid 31528:tid 708] [client ::1:52503] mod_fcgid: stderr: PHP Deprecated:  Array and string offset access syntax with curly braces is deprecated in C:\\htdocs\\joomla-cms\\libraries\\vendor\\wamania\\php-stemmer\\src\\Utf8.php on line 263, referer: http://localhost/joomla-cms/administrator/index.php?option=com_finder&view=indexer&tmpl=component
[Sat Dec 21 13:51:19.834993 2019] [fcgid:warn] [pid 31528:tid 708] [client ::1:52503] mod_fcgid: stderr: PHP Deprecated:  Array and string offset access syntax with curly braces is deprecated in C:\\htdocs\\joomla-cms\\libraries\\vendor\\wamania\\php-stemmer\\src\\Utf8.php on line 604, referer: http://localhost/joomla-cms/administrator/index.php?option=com_finder&view=indexer&tmpl=component
[Sat Dec 21 13:51:19.835180 2019] [fcgid:warn] [pid 31528:tid 708] [client ::1:52503] mod_fcgid: stderr: PHP Deprecated:  Array and string offset access syntax with curly braces is deprecated in C:\\htdocs\\joomla-cms\\libraries\\vendor\\wamania\\php-stemmer\\src\\Utf8.php on line 609, referer: http://localhost/joomla-cms/administrator/index.php?option=com_finder&view=indexer&tmpl=component

Based on the description of this vendor library and the lack of updates it looks unlikely that this will be fixed

PHP 7.4 will now deprecate the curly brace syntax for accessing array elements and string offsets and it is expected that support will be completely removed in PHP 8.0.
Ref: https://wiki.php.net/rfc/deprecate_curly_braces_array_access