List of symbols marked [[EXPERIMENTAL]] in the documentation

Edit: This solution no longer works due to changes in the Entity framework.
The "UnderDevelopment" EntityClass no longer exists. It's not part of EntityClassList["WolframLanguageSymbol"] or WolframLanguageData["Classes"] anymore.

---

The symbols that are market [[EXPERIMENTAL]] in the documentation are in their own entity class of the "WolframLanguageSymbol" entity type, which is named "UnderDevelopment".

EntityClass["WolframLanguageSymbol", "UnderDevelopment"]

Here is a list of all the 25 symbols currently (version 10.3) in EntityList[EntityClass["WolframLanguageSymbol", "UnderDevelopment"]] as a list of links to their online documentation

Hyperlink @@@ 
  EntityValue[
   EntityClass["WolframLanguageSymbol", 
    "UnderDevelopment"], {"CanonicalName", "URL"}] // Column

Autocomplete
AutocompletionFunction
CachePersistence
ContentObject
DeleteSearchIndex
DimensionReduce
DimensionReducerFunction
DimensionReduction
DistanceMatrix
Echo
EchoFunction
FindFormula
FoldPair
FoldPairList
LocalObject
SearchIndexObject
SearchIndices
Snippet
TextCases
TextPosition
TextSearch
TextSearchReport
TextStructure
UpdateSearchIndex
WordTranslation

links to their offline documentation

Multicolumn[
 Hyperlink[#, "paclet:ref/" <> #, Appearance -> "DialogBox"] & /@ 
  EntityValue[
   EntityClass["WolframLanguageSymbol", "UnderDevelopment"], 
   "CanonicalName"]]

and their "TypesetUsage" for a quick overview.

Column[
 EntityValue[EntityClass["WolframLanguageSymbol", "UnderDevelopment"], "TypesetUsage"],
 Frame -> All, Background -> {{GrayLevel[0.95], LightBlue}}, Spacings -> 1]

List available as EntityList[EntityClass["WolframLanguageSymbol", "Experimental"]]

In Version 11, the [[EXPERIMENTAL]] tag appears to be a Graphics object. So we can simply look for this object in all the reference page. The downside to this is if the design of this tag ever changes, this solution will need to be updated.

(* location of reference pages *)
dir = DirectoryName[FindFile["ReferencePages/Symbols/Integrate.nb"]];

(* position of all GraphicsBox in an experimental ref page *)
Position[Import[dir <> "FeatureExtract.nb"], GraphicsBox[{__}, __]]

{{1, 2, 1, 1, 1, 1, 1, 1, 1, 3, 1, 1}}

(* extract the experimental tag *)
exptag = Extract[Import[dir <> "FeatureExtract.nb"], {1, 2, 1, 1, 1, 1, 1, 1, 1, 3, 1, 1}];

(* look for this tag everywhere else *)
syms = Monitor[
 Reap[
  Do[
   file = dir <> name <> ".nb";
   If[FileExistsQ[file] && ! FreeQ[Import[file], exptag],
    Sow[name]
   ],
   {name, Names["System`*"]}
  ]
 ][[-1, 1]],
 name
]

{AnatomyForm, AnatomyPlot3D, AssumeDeterministic, BatchNormalizationLayer, 
  BatchSize, BayesianMaximization, BayesianMaximizationObject, 
  BayesianMinimization, BayesianMinimizationObject, CatenateLayer, 
  ChannelBase, ChannelDatabin, ChannelListen, ChannelListener, 
  ChannelListeners, ChannelObject, ChannelPreSendFunction, ChannelSend, 
  ChannelSubscribers, CloudExpression, CloudExpressions, 
  ContentFieldOptions, ContentLocationFunction, ConvolutionLayer, CreateChannel, 
  CreateCloudExpression, CrossEntropyLossLayer, DeconvolutionLayer, 
  DeleteChannel, DeleteCloudExpression, DotPlusLayer, DropoutLayer, 
  DynamicImage, ElementwiseLayer, EmbeddingLayer, EntityStore, 
  FeatureDistance, FeatureExtract, FeatureExtraction, FeatureExtractor, 
  FeatureExtractorFunction, FileNameForms, FileSystemMap, FileSystemScan, 
  FindChannels, FlattenLayer, FormControl, GalleryView, HandlerFunctions, 
  HandlerFunctionsKeys, InitialEvaluationHistory, MaxTrainingRounds, 
  MaxWordGap, MeanAbsoluteLossLayer, MeanSquaredLossLayer, NetChain, 
  NetDecoder, NetEncoder, NetExtract, NetGraph, NetInitialize, NetPort, 
  NetTrain, Pagination, PoolingLayer, RemoveChannelListener, ReshapeLayer, 
  ResourceData, ResourceObject, ResourceRemove, ResourceSearch, 
  SearchAdjustment, SearchQueryString, SearchResultObject, SkinStyle, 
  SoftmaxLayer, SummationLayer, TargetDevice, TotalLayer, URLSubmit, 
  $AllowExternalChannelFunctions, $ChannelBase, $CookieStore, $EntityStores}

Length[syms]

84

$Version

"11.0.0 for Mac OS X x86 (64-bit) (July 28, 2016)"

Not a terribly fancy solution, but it works.