All TinyMCE 6 setting names A-Z (as far as I can tell)
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Source: https://github.com/tinymce/tinymce-dist/blob/master/tinymce.d.ts
See also: https://www.tiny.cloud/docs/configure/ (for TinyMCE5 but still applies)
Note that this only includes root-level settings, no sub-settings. 

a11y_advanced_options
add_form_submit_trigger
add_unload_trigger
allow_conditional_comments
allow_html_data_urls
allow_html_in_named_anchor
allow_script_urls
allow_svg_data_urls
allow_unsafe_link_target
anchor_bottom
anchor_top
auto_focus
automatic_uploads
base_url
block_elements
block_formats
block_unsupported_drop
body_class
body_id
boolean_attributes
br_in_pre
br_newline_selector
branding
browser_spellcheck
cache_suffix
color_cols
color_map
content_css
content_css_cors
content_langs
content_security_policy
content_style
contextmenu
contextmenu_never_use_native
convert_fonts_to_spans
convert_urls
custom_colors
custom_elements
custom_ui_selector
custom_undo_redo_levels
deprecation_warnings
directionality
disable_nodechange
doctype
document_base_url
draggable_modal
editable_class
element_format
elementpath
encoding
end_container_on_empty_block
entities
entity_encoding
event_root
extended_valid_elements
external_plugins
file_picker_callback
file_picker_types
file_picker_validator_handler
fix_list_elements
fixed_toolbar_container
fixed_toolbar_container_target
font_css
font_family_formats
font_size_classes
font_size_formats
font_size_legacy_values
font_size_style_values
forced_plugins
forced_root_block
forced_root_block_attrs
format_noneditable_selector
formats
height
hidden_input
icons
icons_url
id
iframe_aria_text
iframe_attrs
images_file_types
images_replace_blob_uris
images_reuse_filename
images_upload_base_path
images_upload_credentials
images_upload_handler
images_upload_url
indent
indent_after
indent_before
indent_use_margin
indentation
init_instance_callback
inline
inline_boundaries
inline_boundaries_selector
inline_styles
invalid_elements
invalid_styles
keep_styles
language
language_load
language_url
line_height_formats
max_height
max_width
menu
menubar
min_height
min_width
mobile
model
model_url
move_caret_before_on_enter_elements
newline_behavior
no_newline_selector
non_empty_elements
noneditable_class
noneditable_regexp
nowrap
object_resizing
padd_empty_block_inline_children
paste_as_text
paste_block_drop
paste_data_images
paste_merge_formats
paste_postprocess
paste_preprocess
paste_remove_styles_if_webkit
paste_tab_spaces
paste_webkit_styles
placeholder
plugin_base_urls
plugins
preserve_cdata
preview_styles
promotion
protect
readonly
referrer_policy
relative_urls
remove_script_host
remove_trailing_brs
removed_menuitems
resize
resize_img_proportional
root_name
schema
selector
self_closing_elements
service_message
setup
sidebar_show
skin
skin_url
smart_paste
statusbar
style_formats
style_formats_autohide
style_formats_merge
submit_patch
suffix
table_tab_navigation
target
text_block_elements
text_inline_elements
text_patterns
text_patterns_lookup
theme
theme_url
toolbar
toolbar1
toolbar2
toolbar3
toolbar4
toolbar5
toolbar6
toolbar7
toolbar8
toolbar9
toolbar_groups
toolbar_location
toolbar_mode
toolbar_persist
toolbar_sticky
toolbar_sticky_offset
typeahead_urls
url_converter
url_converter_scope
urlconverter_callback
valid_children
valid_classes
valid_elements
valid_styles
verify_html
visual
visual_anchor_class
visual_table_class
void_elements
whitespace_elements
width
