GeSHi has a deprecated constructor in function require_once()

I had this too, but we can not fix it, because it is in geshi library, not in the drupal module, which is still compatible with php 4. The better thing we can do is create a path from same dir as geshi(it can be in other places, so we can not use a path from drupal root as it will not work for everyone) and add a note in the readme and the project page to this issue, but we can not fix it.

I see now while looking for the issue. Geshi library has a new version 1.0.9, please use this version, it fixes this issue. I will put a note on project page.

Updating the GeSHi library to 1.0.9.0 now gives me the: "Undefined index: LANG_NAME in GeSHi->get_language_name()" in Drupal 8

Seems like there are some API changes which has to be addressed, like:
$geshi->set_language_path($dir); is deprecated now.

@hugronaphor please go to /admin/config/content/formats/geshifilter and click on "Flush geshi language definition cache", if this does not fiz the error please provide the full error message, with file/line number. We do not have this error in a new instalation, so maybe it is something that is on cache.

I'm afraid it's still the same:

Notice: Undefined index: LANG_NAME in GeSHi->get_language_name() (line 614 of libraries/geshi/geshi.php).
GeSHi->get_language_name() (Line: 179)
Drupal\geshifilter\GeshiFilter::getAvailableLanguages() (Line: 207)
Drupal\geshifilter\GeshiFilter::getEnabledLanguages() (Line: 109)
Drupal\geshifilter\Form\GeshiFilterSettingsForm->buildForm(Array, Object)
call_user_func_array(Array, Array) (Line: 514)
Drupal\Core\Form\FormBuilder->retrieveForm('geshifilter_admin_settings', Object) (Line: 271)
Drupal\Core\Form\FormBuilder->buildForm('geshifilter_admin_settings', Object) (Line: 74)
Drupal\Core\Controller\FormController->getContentResult(Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 582)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 151)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 68)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 99)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 78)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 50)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 664)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

I use PHP 7.0.28

Same here, but using 7.x, and running PHP 7.1.x.